header has gone missing

We have a website made by a pro and I’ve been given the admin job. I’m a newbie to WordPress and I wanted to add a link to the header. The admin site is very restricted and does not allow me to make changes to the template so I was looking at the files through my ftp site. When I went to look at the website the header was missing from all pages and showed only text and internal pictures. When I logged into the admin site the pages are missing from the list. It show a count of 40 pages but they can’t be seen on the list.
I have no idea what happened since I didn’t make any changes to code, just looked at the code.
I can’t give the site because we have replaced it with the old site which still works.
Any help would be most appreciated.